1. Field of the Invention
The present invention relates to an improvement in a structure of a canister used in an evaporative fuel emission control system of an internal combustion engine for a motor vehicle.
2. Description of the Prior Art
There has been known a control system for the capture of evaporated fuel from a fuel tank or a float chamber of a carburetor to reduce HC emissions. For adsorbing the fuel vapors, a canister filled with a mass of adsorbent substance such as activated carbon particles is used in the fuel evaporative emission control system.
One type of such canister for an automotive engine is disclosed, for example, in Japanese Patent Application Laid-Open No. 77923/78. In this device, an adsorbent substance comprising activated carbon particles is charged in a vessel 1 as shown in FIG. 1, to form an adsorbent layer 4. The layer has embedded a deflector 14 of conical frustum shape including a bottom 14a positioned against an end portion of a vaporized fuel inlet tube 12.
Meanwhile, in the illustrated device of the prior art, a check valve 16 for introducing air in order to purge the vaporized fuel into the interior of the adsorbent layer 4 is mounted on the underside of the bottom 14a of the deflector 14 and a purging chamber 11 is formed at the bottom of the vessel 1.
The check valve 16 which is opened by utilizing the vacuum produced in an intake manifold of the engine is independent of a port 11a formed in the purging chamber 11 and is maintained in communication with the atmosphere.
Thus, the relation between the pressure tending to open the check valve 16 and the air-flow resistance from the atmosphere to the purging chamber 11 is a problem. That is, when the resistance offered to the air-flow is lower than the pressure tending to open the check valve 16, the check valve 16 is not opened. Then the adsorbent surrounded by the deflector 14 will not be purged. On the other hand, when the resistance is higher than the pressure, the check valve 16 is opened. However, when the resistance is high in this case, the purging ability of the adsorbed fuel will be reduced because of the reduction of the purging air.